Manufacture of chromic acid



atentecl Sept. 26?, lldd trans MARVIN "l'. 'UJDY, 0F NIAGARA FALLS, NEWYORK, ASSIGNOR TO ELECTED METALLUR- GICAL COMPANY, A CORPORATION OE WESTVIRG -I i MANUFACTURE OF CHRONIC ACID No Drawing.

The invention is a process for the manufacture of chromic acid fromferrichrome. The chromic acid. is useful in the electrodeposition ofchromium metal and for other purposes, the ultimate use of the chromicacid being no part of the present invention.

In the process to be described the ferrochrome is dissolvedelectrolytically by connecting it as anode in a sulphuric acidelectrolyte. The cell used for this purpose is equipped with a porousdiaphragm designed to keep the bulk of the chromium compounds in thevicinity of the anode and in the hexavalent state. Portions of theanolyte are removed, either continuously or periodically, and worked forthe recover of chromic acid. The mother-liquor is pre erably treatedfurther for the removal of iron compounds in order that the sulphuricacid content of the anolyte, or more accurately that part of thesulphuric acid which has not combined with iron, may be restored to theprocess.

Sulphuric acid, part of which may be that recovered in the manner justreferred to, is fed to the cathode compartment of the cell in such a wayas to kee conditions in the cell reasonably constant. ecause of theelevated temperatureof the electrolyte, water will evaporate from bothcompartments.

The operation of the cell results in'the transfer of sulphur acid fromthe cathode compartment to the anode compartment but the evaporation ofWater from the catholyte tends to prevent the concentration of sulphuricacid in the catholyte from being diminished. The concentration ofacidand metal compounds, in the anolyte tends to increase continuously.

The cathode is preferably lead. The diaphra in may be made from anyporous, acid- 40 proo material structurally adapted for the purpose,diaphragms composed offiltros or alundum having been used with goodresults. The ferrochrome may contain any proportion of chromium but ahigh content is preferred. An alloy produced by reducing a high gradeore with carbon in the electric furnace gives good results.

The recovery of chromic acid from the ano lyte'is convenientlyaccomplished by crystallization, with such concentration by evapora-Application filed June 2, 1926. Serial n6. 113,335.

course be obtained by suitable evaporation.

To minimize the evaporation required, a high concentration of sulphuricacid is preferably maintained'in the anolyte, though the process isoperable with much less than the preferred concentration of sulphuricacid in the anolyte. The concentration of dissolved material in theanolyte should of course not be permitted to increase to the point wheresolid compounds begin to crystallize in the cell.

The iron can be removed from the anolyte by crystallization, best afterthe bulk of the chromic acidhas been crystallized and removed. Thus 7 0%or more of the CrO can be recovered in a single crystallization, and 90%or morecan be recovered-in two crystallizations with interveningconcentration by evaporation.- A recrystallization for the removal ofiron will remove to of that present. It will be apparent that thesulphuric acid can be suficiently separated from the metallic compoundsto permit the return of the acid to the process by introducing it intothe cathode compartment of the cell from which it was taken or intoanother 7 and the water which is evaporated for purposes ofconcentration and in the cell.

By Way of illustration it may be remarked that suitable conditionsinclude an electro- 'lyte temperature of about 85 (3., a current densityof about amperes per square.

foot, and a potential of about 4.3 volts. Un-

der these conditions theanode efliciently (by which is meant thequotient of the total chromium dissolved by the electrochemicalequivalent of the current passed) ma be. as 5 high as 100% whenferrochrome ano es containing about 68% of chromium are used.

Throughout the process a small quantity of chromium tends to be reducedto the trivalent state. This occurs during evaporam tion and also in thecathode compartment because of thefact that the mother-liquor introducedinto the cathode compartment still contains some chromium. The reducingchromium can be kept below any chosen is maximum by usin auxiliary cellswith lead or other insolu le anodes to reoxidize it to the hexavalentstate. Trivalent chromium is preferably eliminated as far as possiblebefore crystallization as it interferes 20 with the recory of thechromic acid.

I claim: 1. Process of manufacturing chromic acid which compriseselectrolytically dissolving chromium from a ferrochrome anode in the aanode compartment of a diaphragm cell with an anolyte strongly acid withsulphuric acid and substantially free from alkali metal and alkalineearth metal compounds; and recoveringPchromic acid from the anolyte.

2. rocess of manufacturing chromic acid a which compriseselectrolytically dissolving chromium from a ferrochrome anode in theanode compartmentof a diaphragm cell with Pan anolyte strongly acid withsulphuric acid, as imd recovering chromic acid from the anoyter.

3. Process of manufacturing chromic acid which comprises aelectrolytically dissolving chromium from a ferrochrome anode in the ananode compartment of a diaphragm cell containing sulphuric acidelectrolyte; drawing ofi anolyte at a density of 40 B. to 45 1%.; andcrystallizing chromic acid therefrom. In testimony whereof, I aiiix mysignature. 4a MARVIN J. UDY.

